puredata.info has a valid CAcert.org SSL site certificate suitable for https connections and other purposes.
The CAcert.org root certificate, however, may or may not not be present in your system. If it is not, you'll need to manually import it to get puredata.info's certificate fully recognized.
The CAcert wiki has detailed instructions on how to import the root certificate into different systems (including Mac OS X, Windows, Linux, Symbian, Java, Android phones, Acrobat Reader and Palm Pre). There is also plenty of info on importing into different web browsers and some email clients.